Industrial door/gate motor operators distinguish themselves from residential garage door operators by using three pushbuttons, open, close, and stop. Called a three-button station their operation would seem to be obvious but there are variations. Automatic operation, termed “momentary”, requires just a momentary press of the open or close button to move the motor operator to its limit of travel. Momentary operation requires a safety device such as a safety edge or photo-eye so as not to crush something in the opening. Non-Automatic operation, termed “constant”, requires constant pressure on a pushbutton to move the motor operator to its limit of travel. Constant operation requires that all three-button stations be next to the entryway and that releasing the pushbutton will immediately stop the operator. Further distinction between residential and industrial motor operators is that of output torque, industrial operators are those that exceed 100-lbs of force, and such distinctions are in U.L. Specification 325.
Single button operation is a rarely used option but available for industrial motor operators. If the entry is fully open, pushing this button will close it. If the entry is fully closed, pushing this button will open it. If the entry is actively closing, pushing this button will cause it to stop for a moment and then re-open. This is termed, an “Auto” function and is different from residential door operators. Residential motor operators have only a single button that delivers the sequence, opening-stop-closing-stop and the cycle repeats. A quick glance at the sequence shows that whenever the door, stops between the limits, either opening or closing will follow with equal certainty. If the person standing at the button walks away, the next person attempting to enter a partially open door may press the button and get an unexpected closing, followed by an unexpected stop. Rapidly pressing the pushbutton during an emergency gives a revolving roulette wheel of commands and three out of four are wrong. Industrial motor operators command hundreds or even thousands of pounds of force and uncertainty about their direction of movement is bad. Therefore, the single button auto function in industrial motor operators should not include the ability to stop the operator in a partially open position.
There usually are numerous pushbuttons, radio controls and pull cords in operation on one motor operator at one time and conflicts occur regularly. If one person is pressing a close button on one side of an entryway, while at the same time another person is pressing an open button on the other side of the entryway, the motor operator must prefer the open command. The occupant entering has priority over those leaving an entrance. In addition, the closing function is to some extent more hazardous than the opening function. Pressing a stop button, even for a moment, overrides the continuous pressing of either an open or a close button. A shorted button, stuck radio control, blocked photo-eye can issue a continuous command to the motor operator to move in a direction. A continuous command to move might force a person to stand at the stop button, holding it, to prevent movement. This does not allow a responding person to give aid to potential victims. Trapped at the pushbutton station he can only call for someone to turn off power. Therefore, the stop function should latch until all buttons are released everywhere in the system.
In general, the person standing at the entryway will always be able to interpret a safety hazard better than any safety sensor or computer controlled motor operator. The person responding to an emergency will not be skilled in motorized operators. Assuredly, they will not have time to read the manual, safety stickers, or interpret alarms and flashing lights. They are likely to be just a passerby rushing to the aid of someone in trouble at the door or gate. Therefore, the Open Close and Stop buttons must always perform as stated and not change their functions.
Motor operators must have a fully open and a fully closed position setting most commonly implemented by two limit switches and a rotating threaded shaft with non-rotating threaded nuts. The threaded shaft rotates as the door/gate moves by a mechanical linkage driving the threaded nuts linearly. Thereby every position of the door/gate has an exact proportional position of the nut on this shaft. At the limit of travel, the nut presses against a limit switch that signals the motor operator to stop moving in that direction. Limit switches are commonly of the, “normally closed” type, which open their contacts when the threaded nut presses on their lever. This configuration allows that if contact is lost, the motor will not even begin to operate in that direction, indicating a defective or disconnected switch. This is an important safety feature when commanding thousands of pounds of force.
The safest method of obstruction detection is the sensing edge that attaches to and travels with the edge of the moving load. Other fixed, non-moving means of detection such as photo-eye beams, ultrasonic detectors, infrared or motion detectors all have dead zones and blind spots. Motor operator torque detectors using speed, current, chain tension, etc. all depend on a smooth running load because a torque dip follows a torque spike and during the dip, obstruction-sensing force is huge. Force applied along a sensing edge is independent of motor load and there are no dead zones. A sensing edge makes an electrical contact by touching an object signaling the motor operator to immediately stop and then open. Using such devices requires a new operator positional limit in addition to the standard “close limit” and “open limit”, called the “snow Limit”. Historically named, because a buildup of snow activated the sensing edge too early; before actually reaching the motor operators close limit. In fact, even when there is no snow, it is impossible to close an entryway so that it will seal tightly without first pressing its sensing edge. Therefore, at or past the snow limit, the sensing edge signal no longer reverses the motor operator, but just stops it.
The snow-limit distance, as stipulated in standards, is 2-inches before the fully closed position. During the final 2-inches of travel, the sensing edge will just stop the operator thereby trapping anything it stops on and pressing on it with considerable force. Even so, the two-inch standard seems to be reasonable in that even if a child were to press the close button and then lie down in the doorway to see what develops he will project more than 2-inches. Any other living thing less than 2-inches in height are not likely to be able to complain about the experience. Nevertheless, if this snow limit were to drift to 4-inches a serious safety hazard would exist. The operator could stop trapping a person under it with the full force of both the door and the motor operator pushing on him. It is therefore important that the snow limit never exceed 4-inches from the fully closed position.
Installers typically test the operation of each sensing edge by using a tool called a “two by four” placed between the sensing edge and the fully closed position. The motor operator optimally causes the sensing edge to stop on the 1½-inch side and then in a second test, stop and open on the 3½-inch side. Passing this test means that the motor operator's snow-limit engages 2½ inches from the floor with a tolerance of (+/−) 1-inch to allow for drift or wear. Mechanically the tolerance from the snow-limit switch to the close-limit switch is hard to adjust and critical to safety. The threaded limit shafts length, typically 5-inches, proportions to a 20-foot door/gate, or a ratio of 5:240 inches, such that 1-inch at the entryway equals 0.020-inches on the threaded shaft. Therefore, the snow-limit switch lever must be located 0.050-inches before the close-limit switch lever at a tolerance of +/−0.020-inches. In practice this is hard to achieve and harder to maintain over time as the various mechanical components wear.
Reversing the direction of a motor operator while, it is still rotating places a strain on its bearings, windings and metal components that is hundreds of times greater than its normal static load. Some single-phase motors will not reverse direction at all unless they come to a complete stop and continue to run in the original direction at full torque. Therefore, it is desirable to allow the motor operator to come to a complete stop before reversing direction. A simple timer set for one or two seconds whenever reversing direction can allow the motor to coast to a stop before reversing. Unbalanced loads can cause longer coast to stop times by back feeding from the output shaft through the gearbox to the motor. In these instances manufacturers use electrically actuated brakes or special gearboxes to prevent such excessive coasting.
Most industrial motor operators will drive their connected load at velocities less than 6-inches per second. If the moving edge contacts an obstruction, it has more than enough force to move it 6-inches in a second; for example, pressing the top of a persons head even with their shoulder blades. It is critical that any obstruction sensors such as sensing edges, photo-eyes, ultrasonic, or other devices are working prior to using a motor operator. Many but not all obstruction sensors are “monitored”, “fail-safe”, or “supervised” such that if they are not operating correctly, or are disconnected they signal a continuous obstruction and the motor operator will not run. Monitored sensors have two circuits, the monitoring circuit and the sensing circuit. The sensing part is mounted somewhere in the entryway to sense an obstruction while the monitoring part is mounted inside or on the motor operator. If the monitoring part detects the loss of the sensing part it closes a contact, signaling the motor operator to stop operating in one direction.
Industrial motor operators have a rotating output shaft that couples to its load using roller chain and is relatively universal. It can drive its connected load from the right hand side, left hand side, from the front, back, top or bottom and thereby may require differing rotational direction with different installations. For example, opening an entryway could require a clockwise shaft rotation with the motor operator mounted inside the room and counterclockwise rotation if mounted outside the room. Reversing the output shafts rotation involves reversing motor wires and reversing the open-limit, close-limit, and snow-limit switches location on the threaded shaft. If a motor operator manufacturer makes two models for the different rotations, he still must deal with three-phase motors and power lines connecting out of sequence. The installer knows he has the wrong power line sequence or the wrong rotation if he presses the open pushbutton and the connected load closes.
It is critical to know that when the motor is driving the load open, the threaded shaft nuts are traveling toward the open limit switch. Conversely, when closing, the nuts must travel toward the close-limit and snow-limit switches. Incorrect rotation has the entry opening when the threaded shaft nuts are traveling toward the snow and close-limit switches. This is a serious safety hazard as the motor operator will run past the incorrect limit and apply its full torque to the stalled load or the structure holding it. Motor operators thereby should function such that pressing either limit switch, or specifically the wrong limit switch, stops its rotation. This solves one problem but creates another; it becomes possible to have an entryway that opens when pressing the open button but inside the motor operator, it is actually stopping at the close limit switch. The snow-limit function is then missing from the closing cycle and has moved to the opening cycle. Thereby, a closed entryway opens by pressing on the sensing edge or blocking a photo-eye, and the entryway is no longer secure. The installer must insure that the threaded shaft nut is traveling toward the correct limit switch.
The installer usually adjusts the limit switches or threaded shaft nuts while the motor operator has power, and while standing on a 25-foot ladder. Seemingly, no amount of coaxing will get them to stop doing this. During this adjustment, the limit switch will make and brake numerous times until deemed, just right. It is therefore safer if the limits electrically latch such that releasing the limit switches lever does not cause the motor to run.
Connections from pushbuttons to the motor operator use long lengths of low voltage, multi-conductor, unshielded thermostat wire. Nearly every motor operator manufactured uses thermostat type 24-volt controls and wires. It is common that a complete switch wire run totals 1,000-feet. Electronic motor operators do not draw significant current through their switches and therefore do not have wire length limitations but must deal with 1,000-feet of unshielded wire picking up every electrical blip produced by an industrial environment.
It is common wiring practice to disconnect low voltage power from the operator if the motor overheats or when using a manual pull chain. Most stop switches or lock switches simply disconnect 24-volt control power to the operator. Thereby, motorized operators must identify the loss of power as a stop switch signal.
This background description incorporates technical data from the author's knowledge, Underwriters Laboratories specification UL-325, and DASMA, (Door & Access Systems Manuf. Assoc., www.dasma.com) documents. It is a condensed representation of the field of industrial motor operators, is comprised of well-known facts, and well-known functions to those experienced in this subject matter.